1. Field of the Invention
The present invention relates to an image forming apparatus of a copier, a printer or the like, using an electrophotographic method or an electrostatic recording method.
2. Related Background Art
In recent years, attention has been attracted to an image forming apparatus of a cleaning simultaneous with developing type, free from waste toner.
FIG. 6 shows a schematic configuration of one example of image forming apparatus of the cleaning simultaneous with developing type in the contact development using nonmagnetic unicomponent toner.
As shown in FIG. 6, this type of image forming apparatus conventionally comprises a photosensitive drum (image bearing body) 101 rotating along an arrow X as well as a charging roller (primary charger) 102, an exposing tool 103, a developing device 104 and a transferring roller (transferring charger) 105 disposed around the photosensitive drum 101. Besides, downstream of the transferring roller 105 along the conveying direction (along the arrow Kp) of the transferring material P, a fixing device 107 is disposed.
The developing device 104 comprises a developing roller 108 provided in contact with the photosensitive drum 101 for performing the development while rotating along the arrow Y, a supply roller (developer supply means) 109 for supplying a nonmagnetic toner T to the developing roller 108 by the rotation along the arrow Z, a regulating blade (developer regulation means) 110 for regulating the applied amount and the charged amount of the toner T on the developing roller 108 and a agitating member 111 for supplying a toner T to the supply roller 109 and simultaneously agitating the nonmagnetic unicomponent toner. The toner T is a negative polarity toner and a process of sticking this toner T to the exposed portion or a so-called reversal development is performed.
The image forming operation of the above-constructed image forming apparatus will be described below.
In response to an external print signal (image forming signal), the photosensitive drum 101 begins to rotate along the arrow X. First, the surface of the photosensitive drum 101 is charged uniformly by the charging roller 102. The charging roller 102 has the shape of a roller and is rotated by driving means (unillustrated) along the arrow W. Next, an exposure L by means of the exposing tool 103 causes an electrostatic latent image on the photosensitive drum 101 and the electrostatic latent image arrives at the contact part of the developing device 104 by the rotation of the photosensitive drum 101.
In linkage with the above operation, the developing device 104 performs the following operation.
By the sliding friction of the supply roller 109 rotating along an arrow Z and the developing roller 108 rotating along the arrow Y, the toner T agitated by means of the agitating member 111 is supplied onto the developing roller 108. The toner on the developing roller 108, which is given a desired charging amount while the quantity (layer thickness) of the toner is regulated, is carried on the developing roller 108. Besides, the toner in the developing device 104 is agitated by means of the agitating member 111 and carried to the supply roller 109.
When the toner T carried on the developing roller 108 reaches the location in contact with the photosensitive drum 101, a DC development bias is applied to the developing roller 108 by a power supply (unillustrated). Thereby, the toner T carried on the surface of the developing roller 108 is stuck thereto, so that the electrostatic latent image formed on the surface of the developing drum 101 is developed (visualized) as a toner image. At this time, the toner remaining on the surface of the developing roller 108 without contribution to the development is collected into the developing device 104 via the supply roller 109.
In a contact developing unit using a rigid photosensitive drum 101 to perform the cleaning simultaneously with the development under contact with the developing roller 108, the developing roller 108 is desirably a roller made by forming an elastic body into the shape of a roller. Used as this elastic body are those subjected to the resin coating on a solid rubber monolayer or a solid rubber in view of the toner charging property.
Besides, to execute a contact development, a method of using a rigid developing roller to a belt-shaped photosensitive belt is also available in addition to a method of abutting an elastic developing roller 108 against a rigid photosensitive drum 108.
The toner on a photosensitive roller 101 reaches the opposed part of a transferring roller 105 by the rotation of the photosensitive drum 101 and is transferred to a transferring material P by the transferring roller 105. The transferring material P after the transfer of a toner image is conveyed in a direction of the arrow Kp and discharged outside the image forming apparatus after the thermal melting and fixation of the surface toner image by means of the fixing device 107.
On the other hand, the toner remaining (transfer residual toner) on the photosensitive drum 101 without being transferred onto the transferring material P at the time of transfer, passes through the charging roller 102 and reaches the abutting part against the developing roller 108. At this time, the residual toner is collected onto the developing roller 108 under action of a DC developing bias applied to the developing roller 108 and this collected toner is to be supplied to the development at the next image forming.
By repeating the above operation, the image forming of the cleaning simultaneous with developing type is repeated.
In the image forming apparatus of the cleaning simultaneous with developing type shown in the above conventional example, however, there was a problem that a toner had been stuck to the charging roller 102 by long-term use, thus resulting in poor charging. The cause of this is considered as follows.
The image forming apparatus of the cleaning simultaneous with developing type collects the transfer residual toner remaining on the photosensitive drum 101 without being transferred to the transferring material P by means of the developing device 104.
Accordingly, the transfer residual toner has to pass through the charging roller 102. The transfer residual toner at the moment of having passed through the transferring roller 105, however, is a toner comprising the mixing of positive and negative polarities or a broad distribution of rather positive polarity under the influence of a positive polarity transferring bias. Thus, it cannot pass through the charging roller 102 and ends in being stuck thereto for the following reason.
FIGS. 7A and 7B are illustrations of the behavior of the toner near the charging roller 102. As shown in FIGS. 7A and 7B, the transfer residual toner at the moment of having passed through the transferring roller 105 arrives near the charging roller 102 with the mixing of positive and negative polarities or a broad distribution of rather positive polarity, under influence of a positive polarity transferring bias. Here, applied to the charging roller 102 from a charging bias power supply 112 is a negative polarity DC charging bias. And, near the nip part between the charging roller 102 and the photosensitive drum 101, a discharge area is formed by this DC charging bias (B area in FIG. 7A).
Because of mostly comprising a positive polarity toner charged oppositely to the normal polarity, or a so-called reverse toner, the toner having arrived near the charging roller 102 (A area of FIG. 7A) ends in being attracted to the charging roller 102 by the electric field acting between the charging roller 102 and the photosensitive drum 101.
The transfer residual toner stuck to the charging roller 102 enters the B area of a discharge area with the rotation of the charging roller 102. There, as shown in FIG. 7B of an enlarged view of the B area in FIG. 7A, positive and negative charges are generated as a result of discharge, while negative charges are attracted to the side of the photosensitive drum 101 and contributes to charging the surface of the photosensitive drum 101. Besides, positive charges generated at the same time are attracted to the side of the charging roller 102. At this time, since the transfer residual toner is present on the surface of the charging roller 102, the transfer residual toner has grown to be more and more positively charged by sticking of positive charges.
And, even if passing through the nip part and the downstream discharge area, the positively charged transfer residual toner remains stuck to the charging roller 102 and ends in being attracted to the charging roller 102 by the electric field acting between the charging roller 102 and the photosensitive drum 101.
After one turn of the charging roller 102, the toner stuck to the charging roller 102 enters the B region again as it proceeds and is recharged because of being positively charged. Furthermore, a new transfer residual toner brought on with the rotation of the photosensitive roller 101 is further stacked over the toner layer stuck already to the charging roller 102 and charged positively by discharge near the charging roller 102 in a similar manner to the above-mentioned. Like this, the toner stuck to the charging roller 102 is charged still more intensively and at the same time a transfer residual toner is stacked.
As a result, toners are stacked on the charging roller 102 in several layers, so that the photosensitive drum 101 cannot be charged to a normal surface potential and poor charging takes place.
To prevent the poor charging, there has been proposed a method for negatively charging a toner by using an auxiliary material such as toner charging member formed of a brush, a sponge or the like between the transfer roller 105 and the charging 102 so as to negatively charge the toner stuck onto the photosensitive roller 101 immediately after the transfer process.
However, this method had a problem of bringing about a rise in the cost and a lack of stability.
It is an object of the present invention to provide an image forming apparatus using the cleanerless method and the contact charging method.
It is another object of the present invention to provide an image forming apparatus eliminating the occurrence of poor charging.
It is still another object of the present invention to provide an image forming apparatus enabling the toner stuck to a charging member to be transferred to an image bearing body under action of an electric field.
It is yet another object of the present invention to provide an image forming apparatus comprising: an image bearing body for bearing a toner image; transferring means for transferring a toner image on the image bearing body to a transferring material; a charging member for charging the image bearing body having a residual toner after the transfer on a surface thereof by being brought into contact therewith; electrostatic image forming means for forming an electrostatic image on the image bearing body charged by the charging member; developing means for collecting the residual toner on image bearing body while developing an electrostatic image on the image bearing body by using a toner charged in the same polarity as a charging polarity generated by the charging member; control means for controlling a voltage to be applied to the charging member; and a cleaning sequence for performing the cleaning of the charging member by applying a voltage not higher than a discharge threshold and equal to the polarity of the toner after the application of a discharge voltage reverse to the polarity of the toner is applied onto the charging member, when an image formation is not effected.
Further another objects of the present invention would be disclosed in the following description.